Multi-stage plastic extrusion apparatus, comprising essentially two or more extruder units operatively connected in series, provide great latitude and flexibility in operation and performance due to their capability of sequentially imposing markedly different plastic working conditions, such as different screw rotation speeds, in each unit of the composite apparatus independent of the others. Examples of such multi-stage plastic extrusion apparatus and their operation and versatility are provided by the prior art U.S. Pat. Nos. 2,836,851; 3,148,231; 3,261,056; 3,274,647; 3,409,940; 3,467,743; 3,470,534; and 3,979,488.
As is known in the plastic forming art, and as is apparent from the above cited prior art patents relating to multi-stage plastic extruding apparatus, this type of continuous plastic processing and/or continuous plastic shaping device comprises a plurality of at least two essentially individual extruder units arranged in series with respect to each other, with each unit having a separate housing or barrel containing therein a screw which is rotated independently, at least as to speed, from the screw of the other unit or units.
Commonly the extruder screw in each unit of a multi-stage extruder apparatus, is of a different dimension and/or configuration, as well as being designed to operate a distinctive speed, in order for each unit or the screw thereof to uniquely perform a particular or primary function with optimum effectiveness.
Among the various attributes of typical multi-stage extruding apparatus is that the mechanical system thereof is especially amenable to the provision therein of effective venting means whereby gases such as entrained air, water vapor or steam, and formed volatiles, released from the molding compound or plastic stock material being heated and worked therein, can be aptly expelled from the apparatus and its contents. Frequently, such gas venting means for a multi-stage extruding apparatus comprise a simple vent port or opening in the second, or possibly third, stage extruder housing or barrel located upstream from the generally intermediate positioned feed inlet or entry in the housing or barrel for the introduction therein of molding compound or stock. Such conventional gas evacuating vents are illustrated in U.S. Pat. Nos. 2,836,851; 3,467,743; and 3,979,488, cited above. However, highly complex venting systems and means are contrived for some processes or purposes, such as shown in above cited U.S. Pat. No. 3,470,584.
Multi-stage extruders, nevertheless, are subject to certain shortcomings peculiar to their distinctive construction or mechanical system, and modes of operation.
One significant disadvantage of multi-stage extruding apparatus primarily attributable to the distinctive structural designs and independent drive for the separate extrusion screws of such continuous plastic working apparatus, is the difficulty, if not the substantial impossibility, of equalizing or balancing the flow rate or volume of the plastic molding compound or stock material advancing through each unit, one after another, in series, propelled therethrough by their respective screws. This problem or difficulty of equalizing or balancing rates or volume of compound or material flow or movement through each unit in series is frequently compounded because temperatures effect flow properties or rates of such materials, and different or changing temperatures are commonly encountered within the system components due to their specialized role and the different levels of plastic working effected therein.
In some instances multi-stage extruding apparatus, or their sequential flow rates, can be adjusted by means of trial and error measures made during operation so as to approach or achieve a substantial degree of balanced or equalized flow therethrough, or they can be programmed whereby each succeeding unit is set to progressively exceed the flow rate therethrough of the immediately preceding unit to thereby preclude a "bottleneck" effect or the backing up of material within the system. However, an initial imbalance in flow is substantially unavoidable upon starting operation and during the early stages of running, and such an imbalance will usually continue until an equilibrium of conditions, such as temperatures and the like, is reached. Also, programming the system for progressively greater flow rates in each succeeding unit, retards production capacity and, contrary to designed operating conditions, ultimately diminishes the amount of compound or stock within the system or preceding sections thereof whereby its performance with regard to mixing and plasticizing is impeded or diminished.
An imbalance of flow occurring at any time during operation of the system wherein molding compound or stock is advanced from a preceding unit into a succeeding unit at a faster rate or volume than the subsequent unit is moving such compound or stock therethrough, results in a high back pressure which is converse to the designed mode and conditions of operation. In a typical multi-stage extrusion apparatus, such as those described in the cited prior art patents, and particularly those systems having a gas evacuating vent in a succeeding unit located upstream from a generally intermediate feed inlet as shown in the prior art, the back pressure due to such a flow imbalance, forces molding compound or stock to move in a counterflow direction within the succeeding unit upstream from the generally intermediate inlet or feed entry and into an otherwise unoccupied static area comprising the section of the unit provided for the degassing function. This upstream counterflow movement imposed by the back pressure attributable to such a flow imbalance, therefore thrusts the plastic molding compound or stock beyond the designed and normal flow pattern or path and motivating force of a constantly moving stream of the body of the molding compound or stock propelled along by the screw within the unit and advancing therethrough from the inlet to the outlet.
Once molding compound or stock has been driven counter-current upstream beyond the inlet, entering into the unoccupied degassing area, and thereby departing from the established path or current of flow or movement for material advancing therethrough, the displaced molding compound or stock is very likely to remain for an indeterminate period of time within the static environment of the empty area beyond the moving current or stock of material. For instance, the molding compound or stock in a plasticized or semi-plasticized condition due to heat is frequently sufficiently tacky to adhere to portions of the apparatus located in the upstream static area beyond the influence of the motivating current or force of the constantly moving stream of the molding compound or stock propelled by the screw along the designed flow path from the inlet to the outlet.
A prolonged or unequal retention of many plastic molding compounds or stock materials, especially within the relatively hot confines of an extruder unit, is very likely to have adverse effects upon their physical and/or chemical properties. For example, many plastic molding compounds or stock materials include added heat activatable curing agents or contain polymeric ingredients which, although initially heat softenable or "thermoplastic", are inherently "thermosetting" or curable upon heating to a substantially infusible and intractable condition. Such molding compounds or stock materials must be expeditiously and uniformly handled or processed insofar as mixing and heating to plasticize and blend them immediately preparatory to shaping or molding, or they are subject to "scorching" or precuring which occurrence diminishes or destroys their subsequent plasticity and tractability or capability of plastic flow and compliance to shaping.
Thus, a prolonged retention of any plastic molding compound or stock material within an extruder unit, such as the result of its temporary displacement beyond the constantly moving stream of the compound or stock advancing therethrough into a static area, may detract from or inhibit its subsequent dispersion and integration or blending within the main body of molding compound or stock upon its return thereto because of its longer heat history or attainment of an advanced degree of cure. For instance, particles or bodies of molding compound or stock which have been transformed to an advanced state of cure by whatever mechanism or cause, thereafter substantially maintaining their discrete physical identity, resisting integration and uniform fusing and dispersion or blending with more plastic and tractable molding compound or stock whereby the less tractable particles or bodies thereof are thereafter transported essentially intact through the extruder system or operation along with the overall mass of advancing molding compound or stock. The presence of such discrete and indissoluble or intractable particles or bodies passing through the apparatus carried along within the mass of the molding compound or stock, disrupts the uniform or smooth continuous flow thereof through the die means or other shape imposing continuous molding device, and thereby causes imperfections and/or discontinuities within the product molded therefrom as well as a lack of physical homogeneity therein. Such imperfections or discontinuities, or want of homogeneity may significantly detract from a molded products physical attributes or integrity.
Moreover, this deficiency common to multi-stage extrusion apparatus presents a particularly nettlesome problem in the production of plastic insulation coated electrical conductors or wires which are frequently manufactured with such continuous extrusion molding means. Namely, aside from detracting from appearances and physical attributes, such irregularities in insulating coating or coverings on electrical conductors or wires create sites which are more prone to electrical deficiency or breakdown, thereby precluding the product from meeting standards or diminishing its subsequent service life.